1. Field of the Invention
This invention relates to an apparatus for applying a coating to a web. The invention is particularly useful for applying a precise quantity of a coating to a discrete length of web in automated fashion.
2. Description of the Related Art
Web coating apparatus is widely used in the manufacture of a variety of goods. For example, web coating apparatus is commonly used for making pressure sensitive adhesive tapes, wherein adhesive is applied to a web that serves as a substrate or backing of the tape. Other examples include apparatus for making photographic film, coated paper, magnetic audio, video and data storage tapes, as well as other products.
During the manufacture of coated webs, it is often desired to regulate the flow rate of liquid material applied to the web so that a coating having a generally uniform thickness is achieved. In the manufacture of many goods, coatings are applied by spraying the liquid through nozzles toward the web at a flow rate that is controlled by valves. Such practice is generally satisfactory for webs that are relatively long once the web reaches a constant speed during its travel next to the nozzles. In some instances, solenoid operated fluid control valves or variable flow rate pumps have been provided to automatically increase or decrease the fluid flow rate in proportion to the speed of movement of the web.
In certain other web coating processes, a web is directed over a drum and is overcoated, or flooded, with a quantity of fluid material. Before the web is released from the drum and advances toward a wind-up roll, a blade skives excess fluid from the web so that a smooth, substantially uniform coating is achieved.
Overcoating processes for coating webs are used for the manufacture of orthopedic splinting or casting tape, such as Scotchcast.TM. brand and Scotchcast Plus.TM. brand synthetic casting tape (from 3M). Scotchcast.TM. brand and Scotchcast Plus.TM. brand synthetic casting tape include a porous knit fiberglass backing that is coated with a water curable polyurethane resin.
Many processes that involve the coating of webs include a converting operation that occurs after the web has been coated and wound into an intermediate roll. The converting operation includes a rewinding step, where the intermediate roll is unwound and the coated web is wound to a final roll. The converting operation includes cutting steps, so that a relatively long coated web is cut into relatively small sections as may be appropriate for the end use.
In recent years, there has been increased interest in orthopedic splinting and casting tapes that are available in a variety of colors. The Scotchcast.TM. brand and Scotchcast Plus.TM. brand casting tape, for example, include a resin having appropriate dyes to provide a desired color. Some individuals ask for casting tape with bright colors, while others request casting tapes that are less conspicuous.
Unfortunately, polyurethane resin is tenacious and difficult to clean from the overcoating apparatus and the converting apparatus at the end of a coating operation, causing difficulties when attempts are made to manufacture casting tapes of different color. Moreover, the resin immediately begins to cure when exposed to ambient air because of moisture vapor that is present in the air. For these reasons, cleaning of the overcoating apparatus and the converting apparatus is typically a laborious, time consuming task that often entails the use of hazardous solvents.
Another concern during web coating operations is the amount of tension that is present in the finished, coated web. For example, orthopedic casting tape having a porous knit fiberglass backing is an advantage in that the backing can be stretched as needed when applied to the patient for conforming to the patient's anatomy. During manufacture of such casting tape, it is desirable to wind the tape into a final roll wherein the fiberglass backing as packaged is not stretched to any significant degree. The backing will then be able to be stretched as needed during application as the final roll is unwound in order to facilitate conforming the tape to the patient's anatomy.
The desirability of a coated web that is packaged in roll form in a substantially unstretched condition is not limited to coated webs used in the orthopedic field. For example, it may be desirable to provide a resin impregnated fabric in an unstretched condition for use in constructing or repairing automotive, marine or aerospace bodies so that the backing can be stretched as needed by the user and shaped to an appropriate configuration.
Many conventional web coating and converting operations are carried out by applying tension to the web to keep the web taut. Such processes may be suitable for coating nonelastic webs, but are generally unsatisfactory for coating elastic webs such as orthopedic tape backings. Further, if sufficient tension is applied to the web during the coating operation to stretch the web past its yield point, the web may no longer have physical characteristics that are suitable for its intended used.